Abstract: Base on the mission needs analysis and especial requirement for space target,the paper introduces the principle and method of designing a thermal insulation system,consisted of vapor cooled shield,vacuum adiabatic multilayer,low conductivity support structure.The calculation program was prepared to optimize it.The result shows the size of vapor cooled shield is the key factor which impacts the internal temperature field and the heat load of the cryostat.It presents the optimized size.